1. Field of the Invention
This invention relates to electric leads for transmission of electrical power into or out of a cryostat, and in particular it relates to such power leads which are cooled by cryogen vapor.
2. Background Information
Devices utilizing conventional superconducting materials must be operated at very low temperatures, usually very close to absolute zero. The device is typically immersed in a liquid cryogen contained within a cryostat. Helium, which has a boiling point of about 4 K., is commonly used as the cryogen. Interfacing is required for carrying current to and from the device as well as for monitoring controls or instrumentation in the cryostat. Instrumentation leads typically carry very low current and are dimensionally very small so that heat leak into the cryostat along these leads is not a major concern. However, the transmission of relatively large amounts of current as would occur, for instance, in superconducting magnet applications, require that power leads be designed to minimize heat leakage into the cryostat. Presently, the method for accomplishing this is to build power leads that are internally vapor cooled by the vapor that is boiled off from the heat leakage. These leads typically comprise a cylindrical metal tube containing many hollow conductors, such as braided cooper sleeves, through which the vapor passes. This geometry presents a large amount of surface area per unit volume of conductor and results in efficient transfer of heat to the vapor. These leads can be optimized for a minimum heat leak by sizing the length and conduction area so that the heat leak into the lead at the warm end is zero. Thus, the heat conducted to the cryogen is produced only through Joule heating within the lead. This Joule heating can be considerable, however, as in the case of superconducting magnets where the currents could be hundreds to thousands of amperes.
Each watt of heat entering the cryostat requires about 1000 watts to refrigerate. In addition, the liquid helium required for operation at 4 K., a necessary condition for many superconducting devices, is very expensive.
It is therefore the primary object of the present invention to provide an improved power lead for a cryostat.
More particularly, it is an object of the invention to provide an improved vapor cooled power lead which requires boil-off of less cryogen.
It is also an object of the invention to provide an improved vapor cooled power lead for a cryostat using helium as the cryogen in which at least part of the heat load is taken by a less expensive cryogen.